Quadrature filters may be used in various communication circuits. For example, a quadrature filter may be used as a band-pass filter, or as part of a poly-phase band-pass filter, removing low and high frequencies, but allowing frequencies within a range which includes the interesting components of the signal to pass. A quadrature filter may also be commonly used at baseband frequencies in a Zero-IF (Intermediate Frequency) receiver or transmitter architectures, where it may be implemented as two identical low pass filters, where each filter is driven with the same signal amplitude, but with typically 90° phase difference.
In some implementations, quadrature filters may be implemented as analog circuits (rather than digital circuits), for example, to provide faster response. To provide improved filtering, the gain and/or phase adjustment of quadrature filters may need to be finely tuned. However, fine gain and phase adjustment of analog quadrature filters can be difficult. Furthermore, if the quadrature gain and phase adjustment are not aligned to a high degree of accuracy the reception of certain modulation formats (such as 256 QAM (Quadrature Amplitude Modulation) constellation in accordance with International Telecommunications Union (ITU) J.83-A/B/C specification for cable modulation schemes used for transmission of digital video and audio for different geographies, e.g., used in cable television) will be seriously impaired.
For example, for a very fine adjustment, the change in value between components of the quadrature filter may be very small, and, as a result, may be less than the quoted mismatch of two (e.g., adjacently laid out) components of the quadrature filter. To make matters worse, when each component is switched in and out, the varying on-resistance between different switches may also worsen the problem.